SANAA, Yemen — U.S. aircraft buzzed over the capital of Yemen on Tuesday after the State Department ordered all non-essential personnel out of the embassy and urged all Americans to leave due to to "the continued potential for terrorist attacks."

State has advised non-government U.S. citizens to stay away from Yemen since the country's political unrest erupted in 2011 but the warning was upgraded to urge all Americans to leave immediately because of an "extremely high" security threat level.

The U.S. Air Force transported State Department personnel out of Sanaa early Tuesday. Military personnel remain "on the ground in Yemen to support the U.S. State Department and monitor the security situation," Pentagon press secretary George Little said.

A senior defense official told the Associated Press that 50 to 100 diplomatic personnel were flown out of Sanaa in a C-17 military transport at dawn Tuesday and were taken to Ramstein Air Base in Germany. The official was not authorized to discuss the information publicly so spoke on condition of anonymity.

U.S. surveillance planes circled the skies of the capital on Tuesday morning.

"As staff levels at the Embassy are restricted, our ability to assist U.S. citizens in an emergency and provide routine consular services remains limited and may be further constrained by the fluid security situation," the State Department travel warning said.

The British government also ordered its embassy evacuated "temporarily" and had its shipping in the region along the Red Sea on the highest possible alert.

The Yemen Ministry of Foreign Affairs said it had taken "all necessary precautions" to ensure the safety of foreign missions in Sana'a and suggested the embassy closures would hurt efforts to defeat terrorists.

"While the government of Yemen appreciates foreign governments' concern for the safety of their citizens, the evacuation of embassy staff serves the interests of the extremists and undermines the exceptional cooperation between Yemen and the international alliance against terrorism," the ministry said in a statement.

The shutdown of embassies was instigated by an intercepted secret message between al-Qaeda chief Ayman al-Zawahri and Nasser al-Wahishi, the leader of the Yemen-based al-Qaeda in the Arabian Peninsula, about plans for a major terror attack,according to The Associated Press. The officials who gave the information o the AP spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

The U.S. has pummeled terror leaders in Yemen with targeted drone strikes for at least two years. On Tuesday, Yemeni security officials said a suspected U.S. drone killed four alleged al-Qaeda militants in a volatile eastern province of the country.

The drone fired a missile at a car carrying the four men around 2 a.m. local time, setting it on fire and killing all of them, the officials said.

The U.S. Embassy in Sanaa, along with other Western delegations including the British Embassy, have been closed since Sunday due to the threats. Several foreign embassies have closed their doors in Yemen until at least Thursday in reaction to the threat that is perceived to be at its highest in the final days of the holy-month of Ramadan and going into the Muslim festival of Eid that begins Wednesday.

Despite the reaction among the international community to the apparent imminent al-Qaeda threat, in Sanaa there appeared little visible signs of tightened security across the capital other than the regular checkpoints that dot the city.

People arrive at Sanaa International Airport on Aug. 6 as they prepare to leave Yemen.(Photo: AFP/Getty Images)

Yemen's President Abdo Rabu Mansour Hadi, who recently returned from a visit to Washington, has struggled to maintain security while implementing U.S.-backed military restructuring plans following Yemen's 2011 revolution that brought to an end 33 years of President Ali Abdullah Saleh's rule.

The reformation of Yemen's military, which divided in March 2011 during the uprising, and the removal of Saleh's family members who controlled several powerful units, has caused friction within the air force and army challenging Hadi's authority.

Assassinations of military and intelligence personnel have become widespread in Yemen over the last 18 months since the reorganization of the army and air force began. Improvised explosive devices have also been discovered with increasing regularity this year with a rising number of kidnappings and attacks on oil and electricity lines compounding a sense of lawlessness and highlighting Yemen's weak central government control.

The last known AQAP-linked attack in the capital came in September, when the minister of defense survived a car-bombing that left 12 dead -- one of multiple attempts made on his life.

Since June 2012 the militants have dispersed across several provinces resulting in unprecedented increase in U.S.-drone strikes being carried out across the country.

AQAP has been widely considered al-Qaeda's most dangerous affiliate for several years. The group is linked to the botched Christmas Day 2009 bombing of an airliner bound for Detroit and explosives-laden parcels intercepted aboard cargo flights a year later — both incidents involving al-Asiri's expertise.

President Obama and others have consistently alluded to the weakening of core al-Qaeda in Pakistan — particularly since a Navy SEAL team killed Osama bin Laden in Pakistan two years ago. Obama frequently touts bin Laden's death in his speeches, and has declared that the terror group was "on the path to defeat."

On Monday, officials declined to be more specific about the latest threat.

"What we know is the threat emanates from, and may be focused on, occurring in the Arabian Peninsula," White House spokesman Jay Carney said. "It could potentially be beyond that, or elsewhere."

The State Department on Sunday closed a total of 19 diplomatic posts in the Middle East and Africa, including in Bangladesh, Madagascar, Burundi, Rwanda and Mauritius.

Thomas Sanderson, co-director of the Transnational Threats Project at the Center for Strategic and International Studies, said the balance between overreacting and underreacting is a difficult one to strike.

"Inducing such widespread embassy closings may constitute a public-relations victory for AQAP and the al-Qaeda network generally," Sanderson said.